Method of producing a three-dimensional article having a sandwich structure

ABSTRACT

A method of producing a three-dimensional article having a sandwich structure comprises a deforming step of a mainly flat assembly of a core layer of a thermoplastic foam and at least one cover layer of a fiber reinforced thermoplastic synthetic material into a three-dimensional article. The thermoplastic foam of the core layer is an anisotropic foam. The starting materials are selected in such a way that the glass transition temperature of the starting material of the core layer is higher than the glass transition temperature of the synthetic material of the cover layer.

The invention relates to a method of producing a three-dimensionalarticle having a sandwich structure, comprising a deforming step of amainly flat assembly of a core layer of a thermoplastic foam and atleast one cover layer of a fiber reinforced thermoplastic syntheticmaterial into a three-dimensional article.

Such a method is known in the art, for example from EP 0 264 495. Thisknown method comprises laminating at least one cover layer of a fiberreinforced synthetic material, for example a fabric of aramid fibers,which is impregnated with polyetherimid, to a finished foam core likepolymethacrylimid. In a preferred method a combination of startingmaterials is used, of which the synthetic foam has the lowestglass-transition temperature. In order to improve the bonding between acover layer and the foam core, the face of the foam to be laminated, isprovided with shallow grooves, to which a bonding layer from the samesynthetic material is applied, serving as the synthetic matrix for thefiber reinforced cover layer. Hereafter the assembly is subjected to adeforming step, e.g. by means of direct or indirect heating in asuitable mould.

In particular, this method can be used for the production ofthree-dimensional articles for application in the field of aircraft andspacecraft, because of the profitable combination of strength propertiesand relatively low weight.

However, a disadvantage of this known method is the requirement ofrelatively high temperatures for the actual deforming step, e.g. in caseof a cover layer of fiber reinforced polyetherimid, a preheatingtemperature higher than 290° C. and mould temperatures of more than 130°C. to about 180° C., as a result of the selected starting materials. Atthose high temperatures there is a real chance that the foam willcollaps, due to the lower Tg of the foam, which chance is enhanced bythe required deforming pressure. The relation between pressure, time andtemperature during the deforming step is in this way very critical,especially for articles with a thickness of less than 10 mm. This iseven more important, when the glass-transition temperature of the foamis lower. Besides, for some possible applications the bending rigidityof the sandwich panel and therefore of the end-product is insufficient.In order to increase the bending rigidity, the thickness of the foamcould be enlarged and/or one or more additional cover layers could beapplied. Such solutions results always in an increase in weight, whichis often considered as a disadvantage.

The aim of the present invention is to eliminate at least partly theabove mentioned disadvantages.

In particular, the objective of the present invention is to provide amethod of producing a three-dimensionally shaped article having asandwich structure, comprising a three-dimensionally deforming step atrelatively low temperatures.

Yet a further objective of the invention is to provide such a methodwherein the strength properties of the obtained three-dimensionalarticle are improved, in particular the flexural strength and bendingrigidity, compared to the articles which are produced according tomethods known from the state of the art.

Yet another objective of the invention is to provide such a methodwherein the coherence between the applied pressure, time and temperatureduring the deforming step are less critical to the end-product.

The method of the above mentioned type, is according to the inventioncharacterized by an anisotropic thermoplastic foam in the core, and bythe starting materials being selected in such a way that theglass-transition temperature of the starting material of the core layeris higher than the glass transition temperature of the syntheticmaterial in the cover layer.

In the method according to the invention, an anisotropic foam is used,in other words, a closed cell foam with cells of an oblong shape, thelength of which is at least some times the maximum width of the cell.Such an anisotropic foam has a high compression strength and modulus ina direction perpendicular to the surface of the mainly flat assembly, inother words in the thickness direction and so of the three-dimensionallyshaped end-product. In addition to the high compression strength(modulus), such an an-isotropic foam has a high flexural stiffness,improved impact strength and three-dimensional formability at roomtemperature. Here it is noted that in the laminating method of producinga sandwich panel mentioned in the discussion of the prior art, theapplied finished foam is isotropic with almost bulb-shaped cells. Suchan isotropic foam is not three-dimensionally deformable at roomtemperature.

According to the method of the invention the starting materials areselected in relation to the glass transition temperatures, to ensurethat the cover layer starts to flow, while the foam is not collapsing.The chance of collapsing of the foam in the method as described inEP-A-0 264 495, is almost sure, due to the preferred material choice andrelated deforming temperatures As the glass transition temperature ofthe foam is already reached long before the synthetic matrix material ofthe fiber reinforced cover layers starts to soften. Advantageously themelting point of the synthetic material of the fiber reinforced coverlayer is nearby (±25° C.) the glass transition temperature of thestarting material of the foam core layer, preferably the melting pointis somewhat lower than the glass transition temperature instead of beingsomewhat higher.

The assembly of a foam core layer and at least one cover layer,preferably two cover layers, at both sides of the core layer, can bemade from an anisotropic foam, upon which the cover layer or layers areplaced freely. In order to obtain a mutual bonding, relatively highpreheating temperatures (before the actual deforming step) are required.Advantageously this method is used for relatively thick foam layers,i.e. with a thickness of 12 mm or more, like 25 mm.

Advantageously the method comprises a sandwich production step byin-situ forming of a sandwich panel, which panel is subjected to thedeforming step as the mainly flat assembly. According to this preferredembodiment the sandwich panel, which is produced as an intermediate forthe manufacturing of the three-dimensional article, is formed in onetime in-situ. In other words, the foam forming and bonding of one ormore cover layers to the formed foam is all executed in one step. Thein-situ forming of a panel having a sandwich structure is known as such,e.g. from EP 0 636 463 of the applicant. In general such a methodconsists of the step of placing a film of thermoplastic material, suchas polyetherimid containing an amount of a blowing agent, between twolayers of a fiber reinforced thermoplastic synthetic material, such asglass fiber reinforced polyetherimid. Hereafter this assembly is placedbetween two press plates heated optionally, after which the film isallowed to foam, by applying heat and pressure to the press plates, ingeneral according to a fixed foaming curve, to a predetermined foamthickness. When this foam thickness has been attained, the obtainedsandwich panel is cooled in a controlled way, in general according to acertain cooling curve. In this way, the foam is formed andsimultaneously a bonding takes place between the formed foam and thecover layers of the fiber reinforced synthetic material, resulting in avery strong bonding of the foam to the cover layers. The structure ofthe foam of the sandwich panel which is produced by in-situ, isanisotropic, generally consisting of oblong closed cells, of which thelength is several times the widest cross section, for example 5 times.

In particular this in-situ foaming method is suited to produce thinsandwich panels having an optimal combination of three-dimensionalformability at low temperatures, even at room temperature, high bendingrigidity, low weight and small thickness, e.g. less than 12 mm. Inaddition the deforming conditions, this means the profile of pressure,period of time and temperature is less critical to the quality of theend-product.

The starting material for the foam core layer can be any thermoplasticsynthetic material or mixture of materials, which can be foamed using anappropriate blowing agent. Examples include amongst others,polyetherimid (PEI), polyethersulfon (PES) and polysulfon. In particularpolyethersulfon and especially polyetherimid are preferred, because ofearlier mentioned advantages and also because of the excellent fireresistant properties, which are favourable for applications in thespace- and aircraft industry and also in other transport sectors.Examples of appropriate blowing agents for polyetherimid are acetone andmethylenechloride. Other blowing agents either as solvent or swellingagent or as physical and/or chemical blowing agent or a combinationthereof are known in the art. Mostly the starting material for the foamcore layer is a film, which is impregnated with the appropriate amountof blowing agent. If desired the additives, like nanoparticles, fibersand flame retarding agents can be added. The thickness of the film isnot limited and varies for example from 75 to 400 micrometer or more.Several stacked films can also be applied.

Examples of the synthetic matrix material for the cover layer includepolycarbonate (PC), polymethylmetacrylaat (PMMA) and mixtures ofco-polymers, further for example polyethyleneterephtalate withpolybutyleneterephtalate (PET/PBT) like Valox (General ElectricCompany), and a mixture of PC with PEI. Other fire resistant polymersare also applicable. If desired additives, like flame retardaning agentscan be added. Preferably polycarbonate (PC) is applied as syntheticmatrix for the cover layers, in particular in combination with a PEIfoam. Polycarbonate (PC) has a glass transition temperature of about150° C. and a melting temperature of 220° C., while polyetherimid (PEI)has a glass transition temperature of 220° C. This is a preferredmaterial combination because of the excellent strength properties, fireretarding properties, deformability at low temperatures and therelatively low density of the sandwich panel obtained therefrom andtherefore of the shaped article. It is noted, that a lower deformingtemperature results in a weight benefit, which is especially importantin the aircraft- and space industry. Examples of articles, manufacturedaccording to the method of this invention, comprise covering panels,especially ceiling-and/or side-wall panels, in particular for theinterior walls of spaceships, airplanes and transport vehicles such astrains, trams and busses, complete tubs for seats, arms and/or backs ofseats and seats for seating furniture for means of transport. Loadcarrying constructions could also be produced according to the method ofthis invention. Other typical end products which can be made accordingto the invention include helmets, bath tubs, furniture and certain carparts.

The fiber selection for the fiber reinforced synthetic material is notlimited in any way. Inorganic fibers, such as glass, metallic and carbonfibers and organic fibers including aramid fibers, and natural fiberscan be applied, if they are resistant to the conditions prevailingduring carrying out the method. The fibers may be oriented or not.Knitting, fabrics, cloth and unidirectional fibers are different formsof appearance thereof. Beforehand the fiber structure is in generalimpregnated with a synthetic material into a so-called prepreg, andadvantageously this is consolidated to a cover layer, which is used asstarting material for the method according the invention. Othertechnologies are film-stacking and laminating methods.

Preferably the thickness of the in-situ formed sandwich panel is smallerthan 12 mm, or more preferably smaller than 8 mm, because of thefavourable relation between bending rigidity and weight. When thethickness is larger, the relative profit in bending rigidity and otherproperties is lower.

In order to improve the strength properties one or more intermediatelayers of a fiber reinforced thermoplastic synthetic material may bepresent in the foam. In other words, the final product obtained has asandwich configuration, comprising alternating layers of foam and fiberreinforced thermoplastic material respectively.

Because of the choice of the material for the cover layer,advantageously the deforming step is performed at a mould temperaturebelow 150 C, depending on the materials selection for the mould. Besidesa clear advantage with respect to the operational costs of the method ofproducing, like already argued before, a deforming step at lowertemperatures offers the opportunity to obtain an end product having alower weight.

In this context, it is noted that the deforming step comprises theworking of the surface of the sandwich panel, during which at least apart of this surface is given a different shape.

Hereinafter the invention will be further illustrated by means of thefollowing examples.

EXAMPLE 1

A sandwich panel, consisting of consolidated, glass fiber reinforcedpolycarbonate cover layers and an in-situ foamed foam of polyetherimidis manufactured according to the method described hereinafter.

A film of polyetherimid having a thickness of 250 micrometer and such afilm having a thickness of 125 micron, which films are known as Ultem1000 standard grade from General Electric company, impregnated withacetone, are placed between two cover layers with a thickness ±0,25 mm.Such a cover layer is a consolidated sheet of glass fabric (InterglasStyle 91135), impregnated with 32±1% polycarbonate, manufactured by thefilm stacking method known as such.

This assembly of films and cover layers is placed between two heatedpress plates, to which a pressure is applied of about 25-50 kg/cm2.After the assembly has reached a uniform temperature, the press isopened according to a foaming curve related to the selected type offilm, until the foam thickness, produced from the film, has a value of5,2 mm. After a controlled cooling, the sandwich panel is dried in orderto remove acetone as much as possible.

The in-situ sandwich panel obtained in this way, has an anisotropic foamstructure, having mainly oblong cells with the largest dimension in thethickness direction of the panel. At a foam density of 90 kg/m3 thecompression strength is 2.3 MPa. For comparison: isotropic PEI foamhaving a density of 90 kg/m3, has a compression strength of 1.3 MPa.

The sandwich panel of 220×220 mm manufactured as mentioned above, isplaced between two heated press plates, maintained at T=210-240° C., andheated during 10-20 seconds. To prevent sticking of the softpolycarbonate to the press plates, a 0,5 mm thick blanket of siliconrubber is used as separation film.

The total assembly is placed upon a heated wooden mould (T=50-70° C.)with a hollow circular hole, after which a flat wooden upper mould(T=50-70° C.), with a hole, is placed on top of the assembly. With aconvex synthetic stamp (T=50-70° C.) provided with a handle, thesandwich panel is pressed into the hollow mould, by which the uppermould functions as a sort of clamping device. After about 10 seconds thestamp is removed and the shaped article can be taken out of the mould.The recess in the sandwich panel thus formed has a diameter of 125 mmand a height of 25 mm. The thickness along the section of the recess iseverywhere almost the same, and almost equal to the starting thicknessof the foamed, dried sandwich panel.

EXAMPLE 2

A sandwich panel, consisting of consolidated glass fiber reinforcedpolycarbonate cover layers, and an in-situ formed foam of polyetherimidis produced according to the following method.

Two films of polyetherimid, each having a thickness of 250 micrometer,known as Ultem 1000 standard grade from General Electric company,impregnated with acetone are placed between two cover layers having athickness of ±0,50 mm. The cover layer is a consolidated sheet of 2layers glass fabric (US Style 7781), impregnated with 32±1%polycarbonate, manufactured by the known film stacking method.

This assembly of core films and cover layers is placed between twoheated press plates, to which a pressure is applied of about 25-50kg/cm2. After the assembly has reached the foaming temperature, thepress is opened according to a foaming curve related to the selectedtype of film, until the required foam thickness of 7,2 mm is obtained.After a controlled cooling, the sandwich panel is dried in order toremove acetone as much as possible.

The in-situ sandwich panel manufactured in this way, has an anisotropicfoam structure having a compression strength of 2.1 MPa at a density of90 kg/m3.

In the same way as mentioned in example 1, the above mentionedmanufactured sandwich panel of 220×220 mm is heated and then shaped.

The bowl-shaped recess thus formed, in the sandwich panel has a diameterof 135 mm and a height of 30 mm. The thickness along the section of therecess is everywhere almost equal to the starting thickness of thefoamed, dried sandwich panel.

1. A method of producing a three-dimensional article having a sandwichstructure, comprising a deforming step of a mainly flat assembly of athermoplastic foam and at least one cover layer of a fiber reinforcedthermoplastic synthetic material, into a three-dimensional article,comprising the foam of the core layer being anisotropic, and by thestarting materials being selected in such a way that the glasstransition temperature of the starting material of the core layer ishigher than the glass transition temperature of the synthetic materialin the cover layer.
 2. A method according to claim 1, wherein the methodcomprises a sandwich forming step of in-situ forming a sandwich panel,which panel is subjected to the deforming step as the mainly flat panel.3. A method according to claim 1, wherein the starting material of thecore layer is selected from the group comprising polyetherimid (PEI),polyethersulfon (PES), polysulfon or a mixture thereof.
 4. A methodaccording to claim 1, wherein the starting material of the cover layeris selected from the group comprising polycarbonate (PC),polymethylmethacrylate (PMMA) or a mixture containing such a compound.5. A method according to claim 1, wherein the thickness of the in-situformed sandwich panel is smaller than 12 mm.
 6. A method according toclaim 5, wherein the thickness of the in-situ foamed sandwich panel issmaller than 8 mm.
 7. A method according to claim 1, wherein thedeforming step is performed at a mould temperature below 150° C.
 8. Amethod according to claim 1, wherein the foam of the mainly flatassembly comprises at least one intermediate layer of fiber reinforcedthermoplastic synthetic material.